To keep the list of messages displayed while reading a specific message, you can use a preview panel either to the right of or below the list. Click on or and drag them to modify the location and the size of this panel.
The list of messages can be sorted by each of its columns. Click on a column header to sort according to the content of this column, and click again to reverse the sort order. An icon or next to the column title indicates the current sort method. You can select the default sort method in your mail preferences, which can be found in the Options menu.
The button in the top right corner opens a menu which allows you to change the way the message is displayed, to download it as a text or .eml file, or to display the full header.
If a message has one or several attachments, they are indicated in the list by the symbol. In the page where the message is displayed, the list of attachments is shown in the header. Click on the name of an attachment to open it or download it, depending on its type and on your browser. The button opens a menu which allows you to download all the attachments at once, to display them in the message, or to store them in your documents.
An urgent message is indicated by the symbol in the list of messages.